The research objectives of this proposed investigation are to evaluate in man the action of a variety of pharmacological, hormonal, and environmental influences on the deiodination rate of triiodothyronine in thyroxine by a newly devised double iodine isotopic method. This new method allows the determination of the rapidity of onset, quantitative degree of acceleration or inhibition and duration of action of any influence or agent which may alter the peripheral deiodination rates of these thyroid hormones. It is contemplated that this investigation will be useful in shedding light on the factors which determine the availability of the thyroid hormone at the peripheral tissue deiodinating sites. In turn, the determination of the various factors may allow clarification of the mechanisms by which peripheral thyroid hormone metabolism is altered in various disease conditions.